A nondestructive testing method using pulsed infrared thermal waves is a nondestructive testing technology developed since the 1980s. Based on thermal wave theory as a theoretical basis, the nondestructive testing method using pulsed infrared thermal waves achieves quantitative diagnosis of internal defects or damage of an object, by actively applying pulsed thermal excitation to a measured object, continuously observing and recording object surface temperature field variation with an infrared thermal imaging system, and by testing, acquiring, data processing and analyzing time series thermal wave signals according to modern computer technology and image information processing technology.
An important application of quantitative measurement of nondestructive testing technology by using pulsed infrared thermal waves is to measure defect depth or thickness of a measured object. The defect depth or the measured object thickness is generally calculated by obtaining a certain characteristic time constant of a temperature-time curve. U.S. Pat. No. 5,711,603 uses a derivative peak time of a defect region subtracting a reference region temperature curve as the characteristic time. U.S. Pat. No. 5,711,603 requires that a reference region is first selected, which is difficult to achieve in some applications and introduces errors. A thermal contrast peak method uses the peak time of the defect region subtracting a reference region temperature curve as the characteristic time. However, the peak time is highly affected by factors such as defect size and so on, and also requires a reference region. S. M. Shepard uses the second-order peak time of a temperature-time logarithmic curve as the characteristic time. The method of S. M. Shepard has an advantage that the corresponding peak time is at a relatively early time and is less affected by three-dimensional thermal diffusion. However, the method of S. M. Shepard has a drawback that the second-order differential peak time is largely affected by noise. In the above noted several methods, the defect depth or thickness and the obtained corresponding characteristic time value have determined theoretical relational expressions. A logarithmic deviation time method uses a separation time of the defect region and non-defect region in a temperature-time logarithmic curve as the characteristic time. The logarithmic separation point method also requires a reference curve, and at the same time, it is relatively difficult to accurately determine the separation point. U.S. Pat. No. 6,542,849 selects an approximately linear region from a temperature-time curve, which is fitted to derive its slope. And then defect depth is finally derived by curve fitting with a temperature decreasing theoretical formula. X. Maldague performs a Fourier transform to temperature-time curves, subtracts a reference curve, and uses the zero value time as the characteristic time value.
The above noted several methods in the prior art require a reference curve or a second-order differentiation. Each method has its own different application area.